tests: build: rust: output CONFIG_RUST failure to DESC

Make sure the problems with getting CONFIG_RUST to be set
are displayed in short summary / patchwork. Previously
we would get a failure and a bare "Link".

Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tests/patch/build_clang_rust/build_clang_rust.sh

@@ -58,7 +58,7 @@ prep_config() {
     config_rust=$(./scripts/config --file $output_dir/.config --state CONFIG_RUST)
 
     if [ $config_rust != "y" ]; then
-	echo Unable to enable CONFIG_RUST
+	echo "CONFIG_RUST not set in generated config" >& $DESC_FD
 	exit 1
     fi
 }